In existing track systems (see FIGS. 1, 2 and 3), the track is generally made up of straight and curved sections which are connected by joints and are often mounted onto a base plate to provide support. The width of the track is substantially constant throughout the straight and curved sections. The carriage or carriages which run on these tracks usually have four profiled bearing wheels which engage on a profile on the sides of the track. Track systems can form both closed and open paths.
A first type of carriage (FIGS. 1 and 2) has four bearing wheels located with their centres at the corners of a trapezium whose dimensions provide a substantially consistent fit when the carriage is wholly on the straight and the curved portions of the track. This design of carriage runs on tracks which include curved sections of a single radius and direction. As this type of carriage traverses a joint between straight and curve, the fit on the track becomes looser, often resulting in a clearance condition. Such clearances may be undesirable, and become less acceptable the larger they are. The optimum positions for the wheels, and the maximum clearance as the carriage traverses the joint between straight and curve can be determined using the formulae accompanying the diagram shown in FIG. 14. For example, for a typical system in which the centres of the inner wheels are 40 mm apart and track a radius of 80 mm, and the perpendicular distance between inner and outer wheels is 50 mm, then the maximum clearance would be 0.31 mm. In many applications this would be too much. These clearances become even larger if the radius of the curved sections is smaller, and if any of the dimensions of the trapezium formed by the centres of the bearing wheels increase. In practice, the need to limit the clearances has the effect of limiting the track shapes which can be made, and also minimising the separation of the bearing wheels with a consequent limitation of the moment load capacity of the carriages.
A second type of carriage (FIG. 3) has two pairs of bearing wheels, each mounted on a sub-plate which is then connected by a pivot to the main carriage plate. These sub-plate assemblies move to allow the carriage to travel around tracks with left handed and right handed curves, and curves of differing radii, with very little clearance between bearing wheels and the track. The additional parts and complexity of the sub-plates and swivel bearings makes these carriages more expensive than the first type, and they are not suited to traversing track systems with very tight bend radii. The sub-plates and bearings may also result in increased friction between the bearing wheels and the track.
NL 1 001 452-C discloses a track and carriage system for reading barcodes. The carriages have a single bearing wheel on one side of the track and a pair of bearing wheels on an opposite side of the track. Although the draughtsman appears to recognise that changes in track geometry might be desirable, the profile of the sides exhibit sharp changes in radius which would be undesirable, and there is no further reference to the shape, which is not a key feature of the patent specification.
Tracks and carriages are also used in overhead rail systems where the track forms a rail which varies in a vertical plane, and the wheels run along the upper and lower surfaces of the rail. In such systems the weight of the carriage and any load which it carries always act in a downward direction helping to maintain contact with the rail.
Japanese Utility Model No. 5-54601 discloses an overhead rail system in which the carriage has bearing wheels which run along the upper surface of the rail and a single guide wheel which contacts a lower surface of the rail. At curved sections the upper surface of the rail is cut away so that the rail becomes thinner, but this would not allow close contact to be simultaneously maintained with the upper and lower surfaces of the rail, particularly if the upper surface lies on the outer radius of a curve (FIG. 2).
GB 2 261 863-A discloses a conveyor system having powered trolleys mounted on an overhead rail of varying thickness. In this proposal a single drive wheel operates against an upper surface of the rail and a pair of spaced bucking rollers are provided with pressure means (i.e. spring loaded) to ensure adherence of the drive wheel with the rail. A major disadvantage of such an arrangement is that the spring loading increases friction between the carriage and the rail. The document states: “Thanks to the particular form of the rail the trolleys need pressure springs with limited travel and hence reduced dimensions, their extension being included between the compression position on the sections 21 and the compression position on the sections 20. In addition, between said two positions it is not necessary that the spring have approximately constant thrust but instead said thrust is advantageously variable to fall to the minimum on the horizontal sections, thus reducing the rolling friction.” Thus, notwithstanding the variable geometry of the rail, variable spring loading is essential to maintain low friction.
A similar suspended rail and carriage arrangement is disclosed in JP 3 070 670-A, in which the carriage again has a single drive wheel on an upper surface of the rail and a pair of spring loaded wheels which maintain contact with a lower surface of the rail. In curved parts of the rail at ascending and descending sections the radius of curvature of the lower surface is changed by a predetermined amount relative to the radius required for a uniform thickness. This is well-documented form of geometry which, for certain applications and particular combinations of carriage and track size could be a reasonable approximation, but in the absence of spring loading, the clearance between the wheels and the track would be relatively high.
The present invention seeks to provide a new and inventive form of track system in which the carriages can have a large transverse moment load capacity and relatively free movement with a minimum number of components, and which provides considerably greater flexibility in the layout of the tracks.